Commonly-owned U.S. Pat. No. 4,550,663 (DeViaris, 1985) discloses a vehicle for transporting people between horizontally displaced locations. The vehicle is supported by air-cushion pads in a guideway and is drawn by a motor-driven cable. The cable is supported in the guideway by plastic (electrically nonconductive) guide sheaves, each of which is supported in the guidewway by metal (electrically conductive) brackets. the cable moves at speeds in excess of 25 mph.
By applying a voltage to the cable it is possible, among other things, to determine whether the cable has jumped off of a guide sheave, because if the cable contacts a bracket, a circuit would be completed. In this case, the vehicle would be automatically stopped.
In the event that the cable drive sheave on the motor is lined with a nonconductive material, which is typical of many cable drive applications for which the present invention is pertinent, voltage cannot be applied to the cable at the drive sheave. Therefore, it has been known to urge a longitudinal brush having metal bristles radialy against the cable to apply a voltage to the cable. This approach is limited to slow moving cables because of friction between the brush and the cable, poor tracking of the brush on the multi-helical surface of the cable, removal by the brush of the lubrication from the cable, and cable abrasion by the brush.
Alternatively, it has been known to urge a metal idler sheave against teh cable for applying the voltage. A sheave tracks the cable better, does not remove the cable lubrication, and does not abrade the cable. However, it has been found that the metal-to-metal action of the idler sheave against the cable generates unacceptably high noise levels.